XPR1 mediates the pancreatic β-cell phosphate flush

Abstract

Glucose-stimulated insulin secretion is the hallmark of the pancreatic β-cell, a critical player in the regulation of blood glucose concentration. In 1974 Dawson, Freinkel and co-workers made the remarkable observation that an efflux of intracellular inorganic phosphate (P<sub>i</sub>) accompanied the events of stimulated insulin secretion. The mechanism behind this ‘phosphate flush’, its association with insulin secretion and its regulation have since then remained a mystery. We recapitulated the phosphate flush in the MIN6m9 β-cell line and pseudoislets. We demonstrated that knockdown of XPR1, a phosphate transporter present in MIN6m9 cells and pancreatic islets, prevented this flush. Concomitantly, XPR1 silencing led to intracellular P<sub>i</sub> accumulation and a potential impact on Ca<sup>2+</sup> signaling. XPR1 knockdown slightly blunted first phase glucose-stimulated insulin secretion in MIN6m9 cells, but had no significant impact on pseudoislet secretion. In keeping with other cell types, basal P<sub>i</sub> efflux was stimulated by inositol pyrophosphates and basal intracellular P<sub>i</sub> accumulated following knockdown of inositol hexakisphosphate kinases. However, the glucose-driven phosphate flush occurred despite inositol pyrophosphate depletion. Finally, whilst it is unlikely that XPR1 directly affects exocytosis, it may protect Ca<sup>2+ </sup>signaling. Thus we have revealed XPR1 as the missing mediator of the phosphate flush, shedding light on a 45-year-old mystery.